Devices employed to monitor and interrupt fluid flow have generally employed buoyant pistons as the timing element to cut-off such flow after a predetermined volume of fluid has passed through the fuse. Examples of such buoyant piston devices are shown in U.S. Pat. Nos. 2,512,190; 2,518,988; 2,592,486; and 2,554,390. Reliance on a buoyant piston configuration necessitates that a hydraulic fuse be constructed, utilizing very close tolerances and a large number of parts. Recently, a fuse has been described (U.S. Pat. No. 4,655,245) which employs a regulating element for building inlet pressure to a value sufficient to develop the power necessary to drive the timing element--eliminating the requirement for timing element buoyancy. The addition of a regulating element (a movable sleeve in the '245 patent) permits the fuse to function more accurately at lower flow conditions as compared with the predecessor, buoyant piston systems. Since the regulating element permits power to develop sufficient to actuate the timing element, the latter can more accurately measure the quantity of fluid as it passes through the valve, generally independent of the rate of flow, such that actuation of the shut-off mechanism is reliable both at high and low flow rates.
The '245 patent employs a basic spool and sleeve mechanism for both the regulation and the timing elements. Thus, with respect to the regulating element, the metering slots are near the outer periphery of the cylinder--resulting in a large circumferential length (and therefore large surface area) which necessitates the use of tight tolerances to achieve accurate metering. Not only are tight tolerances disadvantageous in that they are more costly to produce, but they are also more sensitive to contamination and to variations in temperature. With respect to the timing element, since the spool and sleeve mechanism has to effect a fluid-tight seal between the outside diameter of the spool and the inside diameter of the sleeve, two additional disadvantages result: (i) concern with the concentricity of the spool; and (ii) the requirement to employ O-ring seals or the equivalent to prevent leakage--thereby incurring increased friction, which detracts from accuracy, particularly at low flow rates and temperatures.